Little change to Isaac, but intensification coming; Joyce forms

Tropical Storm Isaac is a large and impressive-looking storm on satellite images, but data from the Hurricane Hunters reveal that Isaac remains a minimal-strength tropical storm with 40 mph winds, as it heads westward across the Eastern Caribbean. An Air Force Reserve hurricane hunter aircraft which completed its mission into Isaac at 8 am EDT found top winds at the surface near 40 mph, and highest winds at their 5,000 foot flight level of 47 mph. A NOAA hurricane hunter aircraft flying at 10,000 feet has found top winds of 47 mph at that altitude. The Hurricane Hunters found a broad area of light winds with a central pressure of 1003 mb. The aircraft did not observe an eyewall trying to form, and recent microwave satellite images also show no signs of an eyewall forming. There does not appear to be much in the way of dry air near the core of Isaac, as seen on water vapor satellite loops, which is a big switch from what we've seen previously. Visible satellite loops show that Isaac has a much more symmetric circular cloud pattern, and has developed a Central Dense Overcast (CDO) of high cirrus clouds, the hallmark of an intensifying storm. These clouds have very cold cloud tops, indicating that the updrafts creating them are quite strong. An analysis of upper level winds from the University of Wisconsin CIMSS shows that an upper-level pattern of outflow supportive of significant strengthening has developed this morning, with an upper-level outflow channel now well-established to the north, and a new outflow channel opening to the south. Radar imagery from Puerto Rico shows some weak low-level spiral bands that are slowing intensifying and becoming more organized.

Figure 1. Morning radar image of Isaac taken from the Puerto Rico radar. Isaac's rain bands are weak, but are starting to take on a more spiraling shape.

Intensity forecast for IsaacIsaac has consistently confounded predictions that it would intensify, but all the potential factors inhibiting intensification seem to have diminished to the point where intensification has to occur. Wind shear is low, 5 - 10 knots, and ocean temperatures are warm, 29°C, and these warm waters extend to great depth, giving the storm a high total heat content to work with. These factors, combined with the favorable upper-level outflow pattern and more symmetric cloud pattern, support intensification, and all of the intensity models except the HWRF model predict intensification of Isaac to a strong tropical storm or Category 1 hurricane by Friday afternoon. The latest 8 am EDT run of the SHIPS model predicts that wind shear will be light to moderate, 5 - 15 knots, for the next five days. The 11 am EDT wind probability forecast from NHC predicted a 34% chance that Isaac will become a hurricane by Friday afternoon, and a 6% chance it will be a Category 2 or stronger hurricane then. By Friday afternoon, Isaac will likely be close enough to Southwest Haiti that the inner core will be disrupted, and the storm will likely be a 45 - 55 mph tropical storm on Saturday and Sunday as it moves over Cuba. Once Isaac pops off the coast of Cuba into the Florida Straits, it will be over very warm waters of 31 - 32°C (88 - 90°F), wind shear will be light to moderate, and the upper-level wind pattern favorable for intensification, with low wind shear due to an upper-level anticyclone over the storm. It will probably take at least 24 hours with the storm's center over water for it to become a hurricane.

Impact of Isaac on the IslandsThe south coast of Puerto Rico should see Isaac's heaviest rains and strongest winds beginning near 8 pm EDT tonight, with tropical storm-force winds of 40 - 45 mph potentially affecting the southwest portion of the island. The San Juan airport may be able to stay open during Isaac's passage, but with delays when spiral bands move overhead.

Heavy rains and tropical storm-force winds should arrive on the southern coast of the Dominican Republic late tonight, and the Santo Domingo airport will probably be closed on Friday. Rainfall amounts of 8 - 12 inches will likely affect the Dominican Republic Thursday through Saturday, creating dangerous flash floods and mudslides.

Isaac is potentially a very dangerous storm for Haiti, where 400,000 people still live outside underneath tarps in the wake of the 2010 earthquake. Heavy rains from Isaac will begin on Friday morning in Haiti, and last through Sunday. Rainfall amounts of 8 - 12 inches are possible, which will be capable of causing extreme flooding on the vegetation-denuded slopes of Haiti. It will be a major challenge to keep those Haitians living outside safe, if rainfall amounts of 5 - 10 inches occur.

Impact on Florida, Cuba, Jamaica, and the BahamasFlorida, Cuba, Jamaica, and the Bahamas are all at risk of receiving flooding rains and high winds from Isaac. The latest set of 00Z (8 pm EDT) and 06Z (2 am EDT) model runs for Isaac are fairly unified for the coming three days, showing a west-northwestward track over Southwest Haiti and into Western Cuba. At the 4 - 5 day forecast period for Sunday and Monday, the models have come into better agreement, and have shifted west some. Our best-performing model, the ECMWF, has now shifted Isaac's path more to the east, but still is the westernmost of the models, predicting a landfall for Isaac near the Alabama/Florida border on Wednesday. While we do still have some models predicting a path up the east coast of Florida, model consensus now favors a path up the west coast of Florida through the Gulf of Mexico. The recent reformation of Isaac's center more to the south supports the idea that Isaac will take a track more to the west through the Gulf of Mexico. Since this now means a final landfall for Isaac in the Florida Panhandle is likely, the storm will probably have an extra day over water, increasing the odds that it will become a Category 2 or stronger hurricane before this final landfall. The NOAA jet is scheduled to fly into the storm this afternoon, to do a large-scale dropsonde mission to aid model forecasts. These missions can improve model forecasts by 10 - 20%, so the model runs that will be available early Friday morning should have increased reliability.

Impact on Tampa, FloridaThe Republican National Convention begins on Monday in Tampa, Florida. The latest 11 am EDT wind probability forecast from NHC gives Tampa a 15% chance of receiving tropical storm-force winds and a 1% chance of receiving hurricane-force winds on Monday. The latest model tracks for Isaac suggests that the trough of low pressure pulling the storm to the north will not be strong enough to give Isaac a northeastward component of motion when it crosses Tampa's latitude. Thus, Isaac will have difficulty making a direct hit on Tampa without passing over a considerable amount of land first, making a multi-billion dollar hurricane disaster in Tampa very unlikely. I put the odds of a mass evacuation occurring during the convention at 1%; a limited evacuation of people in the Tampa Bay area living in mobile homes in low-lying areas is probably about 5 - 10 % likely. I have detailed information on Tampa's storm surge vulnerability in a post from last week.

Figure 2. Morning satellite image of Tropical Storm Joyce.

Tropical Storm Joyce forms in the Central AtlanticThe season's tenth named storm of the year, Tropical Storm Joyce, has formed in the Central Atlantic. Joyce's formation on August 23 puts 2012 in a tie for second place with 1995 for earliest formation date of the season's tenth storm. Only 2005 had an earlier appearance of the season's tenth storm, when Tropical Storm Jose formed at 2 pm EDT on August 22. None of the models show that Joyce will be a threat to the Lesser Antilles Islands, but it may be a storm that will affect Bermuda. It is possible that Joyce will complicate the track forecast for Isaac 4 - 5 days from now, when the storms may be close enough together to interact.

Quoting presslord:Why in the world would anyone listen to the NHC when we have easy access here to any number of anonymous, dysfunctional bloggers without any qualifications or credentials whatsoever?!?!?!?!

This is exactly why this is my sole source for information. I never bother looking anything up or historic data, since I get everything I need right here. It makes no difference to me that their short term 48 hr forecast has almost been spot on in the last two years. It is more important to know that the NHC is wrong the actual COC is further south and all the models are wrong.

Quoting STXHurricanes2012:Or the EURO playing like the GFS did against Debby when the GFS was the only one showing FL and the EURO showing TX! Could the other way around this time if the EURO is trying to do! LOL!!

So far trending last couple of days SEEMS TO INDICATE THAT as Euro has constantly been west of consensus and consensus gradually coming more west - as someone said yesterday - either way does not matter now as only way Isaac "gets out" is at the expense of someone - a more East path would be best case as less time to intensify

I just have to say I find it hilarious that the ECMWF predicts a stronger ridge over florida when HPC shows a trough over southeastern U.S. in 5 days. No way does a system as strong as the ECMWF is depicting make it all the way to TX/LA border. Look at HPC forecast I find it very interesting

the fact that NONE of the models predicted the long WSW motion Isaac did last night has me a lil worried... Let me see if I get this straight: NOW the GFS says Panama City Beach, EURO says LA COAST, AND the new GFDL also shifted WEST to Appolachicola??? hmmm... please correct me if i'm wrong, but I think Mobile, AL to Key West, FL is ALL gonna be affected by Isaac...

Quoting Levi32:ECMWF certainly keeps the worry wide open for the Gulf of Mexico. I still think it looks unrealistically far west, but the eastern gulf coast has a lot of models pointed at it. I still want to see the 0z runs withe G-IV data in them.

You and me both. The models have been trending westward ever since Isaac developed, and it sure will be nice to have all that data and see if that bucks the trend or not.

Hope came up with a general rule that if a tropical disturbance does not develop before it reaches the eastern Caribbean, it will not develop in the central-eastern Caribbean due to accelerating trade winds, but if it is going to develop, will wait to do so until the western Caribbean, or north of it.

Quoting hercj:Levi, have you ever heard of the Hope rule? Named after the late Dr John Hope. If so can you explain it to me.

Hope came up with a general rule that if a tropical disturbance does not develop before it reaches the eastern Caribbean, it will not develop in the central-eastern Caribbean due to accelerating trade winds, but if it is going to develop, will wait to do so until the western Caribbean, or north of it.

Quoting Levi32:ECMWF certainly keeps the worry wide open for the Gulf of Mexico. I still think it looks unrealistically far west, but the eastern gulf coast has a lot of models pointed at it. I still want to see the 0z runs withe G-IV data in them.

Quoting SouthDadeFish: Honestly, with the hesitation in the intensification, I'm leaning towards a slightly more westward track. The only thing that I'm skeptical about for a west track, which may ending up being a huge thing, is that the frictional convergence over the Greater Antilles my try to slingshot this thing off to the N/NE then back WNW. I don't believe the global models have the resolution to predict that. I think the HWRF may, and that may be why it is farther east. Either scenario is plausible for now. I just don't think the G-IV flight data will help the models' interactions with the Greater Antilles.

I also expect a north jog island cross-over when he passes over Cuba that is going to freak out the S FL people for 12 hours before it resumes WNW.

No it is not - that would be around a CAT 4 - Rita like path only a more intense version - 12Z GFS shifted west - now 12Z Euro shifts west - not good for Northern Gulf Coast - let's hope not a trend. As would mean odds of someone getting a very intense Cane goes up. I am still inclined for a Mobile to Pensacola landfall - BUT if models continue to trend west then all bets are off.

Well, I'm certainly glad to hear that. I was wondering if I were in some alternate reality, because that just didn't sound like Grothar. I'm just checking in while working and can't keep a scorecard to stay up to date with all the drama, though. I just want to know when I need to start boarding up, if I do. It's going to take a half hour to kill all the brown recluses nesting in my hurricane panels.

Don't worry, Torgen. When I insult someone they know it. We are dealing with a possible devastating situation. Levi is a man not a boy. I can not speak for him, but I am sure it is not making him happy on either side. He has a lot of important work to do and does not need this diversion. Neither does the blog.

P.S. (I didn't mean to be condescending :) I knew you would catch on to what I was attempting.)

It is about as far from me as Rita was in '05. So basically 40-50mph winds, a knocked over fence, and a few lawn chairs down.

Right now, in statistical terms, the Euro is an outlier in the long-range. It is showing a markedly different solution than most of the other major models (GFS, GFDL, HWRF), and is also rather different than most of the consensus models. I'm watching it to see how it shifts around, but so far, its kinda like the crazy kid standing away from the group screaming "pick me! pick me!".

I think it is feeling that Isaac will get stronger short term (which is why it tries to plow into Hispaniola), then get torn up by Hispaniola and Cuba. Then it stays weak-ish for a bit (for some odd reason) and eventually strengthens before landfall, when it catches another weakness.

Hi Everyone! Looks like I could be in for it here in Tallahassee. Do we feel pretty confident in the forecast since it's been relatively consistent the past few days? When should I call the ball on jumping ship if I need to?

Quoting reedzone:It doesn't make sense for a storm to smash right through a ridge... the weakness is over Florida and that is where Isaac is going to most likely head to. Whether it be the East Coast or West Coast. It's not passing 85W, in my opinion.

Quoting Levi32:ECMWF certainly keeps the worry wide open for the Gulf of Mexico. I still think it looks unrealistically far west, but the eastern gulf coast has a lot of models pointed at it. I still want to see the 0z runs withe G-IV data in them.

Levi, have you ever heard of the Hope rule? Named after the late Dr John Hope. If so can you explain it to me.

Quoting reedzone:EURO is out to lunch this afternoon. A Hurricane cannot slam into a ridge, maybe a banana like ridge, but not the side of a ridge.. Should recurve near Florida. I'm holding off on the REED run until I see better data in the models tonight. Off to work.

ECMWF certainly keeps the worry wide open for the Gulf of Mexico. I still think it looks unrealistically far west, but the eastern gulf coast has a lot of models pointed at it. I still want to see the 0z runs withe G-IV data in them.